1. Field of the Invention
The present invention relates generally to a process for separating scandium from an aqueous solution and, more particularly, to such a process which utilizes fractional liquid-liquid extraction techniques.
2. Description of the Prior Art
As is well known in the art, an aqueous solution such as a tantalum residue is a very complex material containing over twenty different metals. One of the metals present in tantalum residue is scandium. It is recognized by all who work in this area that it would be very advantageous to be able to recover scandium from tantalum residue, since scandium has use in many commercial applications such as in lasers and aluminum alloys. Unfortunately, presently very little information is available on the recovery of scandium from tantalum residue. In fact, the U.S. Bureau of Mines is one of the only sources of publications directed to scandium recovery. Although little is known about recovering scandium from tantalum residue, some information is available regarding the recovery of scandium from other metal sources.
For example, in the late 1950's it was found that scandium poisoned the dodecyl phosphoric acid (DDPA) extractant used for uranium recovery. Researchers found that by stripping the organic with HF, a 10% scandium precipitate with numerous impurities was obtained. To acquire a pure product, the precipitate was digested in NaOH for four hours at 90 degrees C. The mixture was filtered and the residue was digested in HCl. The pH and temperature of the liquor were then adjusted to 4 and 100 degrees C., respectively. Oxalic acid was added under closely controlled conditions to precipitate scandium. By calcining the oxalate precipitate, a 99.5% Sc.sub.2 O.sub.3 product was obtained.
A significant amount of work has been performed on recovering scandium from tungsten bearing material (i.e.--as disclosed in U.S. Pat. Nos. 4,718,995; 4,718,996; 4,808,384; 4,626,280 and 4,624,703). As part of the processes disclosed in these patents, scandium, as well as some iron and manganese are loaded on a mixture of alkyl primary amines (i.e.--Primene JMT diluted in alkyl benzenes). The scandium is stripped from the organic using 1 M to 4 M HCl. The scandium is then recovered by "conventional techniques", i.e. many additional purification stages. Currently, scandium is recovered from tungsten residue by the Mitsubishi Electric Corporation of Japan (refer to Rare Metals, "Preparation and Application of Scandium Compounds", 1990, pages 53 through 56). The organophosphate DEPHA is used to load scandium from a chloride leach solution, and scandium is recovered as ScF.sub.3 with other impurities. The fluoride is then converted to hydrate and dissolved in HCl. This chloride solution is passed through ion exchange to remove Zr, Ti, Hr and Th. Calcium is removed by hydrolysis before scandium is precipitated with oxalate. Scandium oxalate is then roasted above 600 degrees C. to obtain pure Sc.sub.2 O.sub.3.
Another process for recovering scandium from a tungsten source (refer to U.S. Pat. No. 4,898,719) uses thenoyltrifluoroacetone dissolved in an aromatic solvent as the organic. This process reportedly produces a fairly pure scandium product; however, there are significantly fewer impurities in this material than in tantalum residue. Ion exchange methods have also been developed for the recovery of scandium (refer to U.S. Pat. Nos. 4,765,909; 4,816,233; 4,965,053 and 4,968,504). This cumbersome technique was developed for recovering trace amounts of scandium from solution, and is inefficient for solutions containing significant concentrations of scandium.
As can be seen from the foregoing, all the known processes for obtaining scandium from aqueous solutions are complex and time consuming. In addition, all of the aforementioned processes must recover scandium from a precipitated form. Finally, and perhaps most importantly, none of these known processes is capable of recovering scandium from an aqueous solution such as a tantalum residue. Therefore, there is a need for a new process for recovering scandium from ores, residues and leach solutions containing scandium, and in particular for a new process for recovering scandium from tantalum residue.